Move import into try block so we don't break python2.6 without unittest2.
Move import into try block so we don't break python2.6 without unittest2.

chriskuehl/gsutil-debian,GoogleCloudPlatform/gsutil,fishjord/gsutil,dtjackson/gsutil,dimfeld/gsutil,mattdr/gsutil,ttiurani/gsutil,GoogleCloudPlatform/gsutil,BrandonY/gsutil,craigcitro/gsutil